The invention relates to scintillation detectors such as are used in a wide variety applications (e.g. high energy physics, liquid scintillation counters, scintillation camera systems). More particularly, the invention relates to scintillation camera systems which are used to form nuclear medicine images. In its most immediate sense, the invention relates to the detectors which acquire the data used to form nuclear medicine images.
In nuclear medicine, a radioisotope is administered to a patient and the radioisotope is taken up by an organ of interest (e.g. the heart or the brain). As the radioisotope decays, gamma rays are emitted from the patient, collimated by a collimator, and directed to a scintillator (usually a scintillation crystal of NaI(Tl)). The gamma rays interact with the scintillation crystal to form minute flashes of scintillation light ("scintillation events"). Photomultiplier tubes are arranged to detect these scintillation events and to convert them to electrical signals. These signals are then used to reconstruct an image of the patient.
Photomultiplier tubes are not stable; the gains of such tubes change as the tubes age and in accordance with the conditions under which the tubes are operated. It has long been recognized that the performance of scintillation camera systems can be improved by controlling the gains of the photomultiplier tubes. U.S. Pat. No. 4,583,187 discloses a system which does this; the system is marketed under the DIGITRAC trademark by Siemens Gammasonics, Inc., assignee of the present application.
Even though the DIGITRAC system is highly accurate and successful at controlling the photomultiplier tube gain over the long term, short term variations in the performance of the photomultiplier tubes in a conventional scintillation camera system cause the system to be in a detuned state at any particular time. The camera must consequently be periodically retuned. This takes time, requires the services of technical personnel and makes the system unavailable for clinical use. It would therefore be advantageous to provide a system which could maintain a properly-tuned state of a scintillation camera detector.
One object of the present invention is to provide apparatus which can be used to monitor the performance of a system which utilizes a scintillator as a radiation detector. Another object is to provide a system which will maintain a scintillation camera system in a well-tuned state. Still a further object is, in general, to improve on known apparatus of this general type.
In accordance with the invention, at least one light-emitting device is in optical communication with the entrance face of a scintillator, so that light emitted by the light-emitting device passes through the scintillator before becoming incident on, e.g., a photodetector such as a photomultiplier tube.
In accordance with method aspects of the invention, the photodetector array of a scintillation camera is exposed to flashes of simulated scintillation light from fixed locations at the entrance face of the scintillator. The flashes of light mimic scintillation flashes and are processed by the camera as if they were produced by scintillation events. When the detector is in a well-tuned state, the apparent locations of thus-produced surrogate "scintillation events" are registered and used as benchmarks. Subsequent changes in such apparent locations from the registered benchmark locations may then be used to identify, and to correct, de-tuning of the detector.
In accordance with apparatus aspects of the invention, light-emitting devices are incorporated within the detector of a scintillation camera system. In further accordance with the invention, the light-emitting devices are energized to generate surrogate scintillation events which are processed by the scintillation camera system as if they were gamma ray-generated scintillation events. Advantageously, the light-emitting devices are light-emitting diodes ("LEDs") and are located adjacent the entrance face of the scintillator. As is discussed below, by energizing the LEDs, changes in the state of tune of the detector can be identified and the detector can then be retuned to a well-tuned state.
Significantly, the invention differs from apparatus advertised by General Electric under the AUTOTUNE trademark. In the General Electric apparatus, an LED is mounted to each photomultiplier tube and the light output of that LED is made directly incident upon that photomultiplier tube. The gain of each photomultiplier tube (or the corresponding preamplifier) is individually adjusted so as to keep the output from the tube at a predetermined level when the LED is energized. In accordance with the preferred embodiment of the present invention, the LEDs are mounted to the entrance face of the scintillator and the light from the LEDs passes through the scintillator before becoming incident upon the photomultiplier tubes. Furthermore, each LED is in optically operative relationship with more than one photomultiplier tube because light from each LED reaches more than one photomultiplier tube after passing through the scintillator.